1. Field of the Invention
The present invention relates to a converter, and more particularly to a fly-forward converter with an energy recovery snubber.
2. Description of Related Art
The isolated flyback converter topology is widely used for switch mode power supplies (SMPS) due to its low parts count and simplicity. The basic topology is shown in FIG. 1. The converter comprises a coupled inductor L, which has a primary winding La and a secondary winding Lb for each output. A switch S, typically a metal oxide semiconductor field effect transistor (MOSFET), is provided for periodically connecting the primary winding to an input voltage. The/each secondary winding is connected via a diode D to an output capacitor C.
FIG. 2 shows typical current waveforms for the circuit illustrated in FIG. 1. When switch S is closed, current flows in the primary winding. This induces a voltage across the secondary winding which reverse biases the diode. Thus, no current flows in the secondary winding and energy is stored in the primary winding. When switch S is opened, the current in the primary winding drops rapidly, inducing a voltage across the secondary winding which forward biases the diode, so that current flows in the secondary winding until the energy stored in the inductor while switch S1 was closed is transferred.
As can be seen from FIG. 2, there is a commutation interval immediately after switch S is opened, during which some current flows in both windings. While this commutation of current from the primary winding to the secondary winding takes place, the primary current has to be given an alternative path in which to flow, in order to protect the switch.
This alternative path may take the form of a dissipative snubber. However, such snubbers reduce the power conversion efficiency. For example, for a 100W design, a dissipative snubber will reduce power conversion efficiency by between 1 and 3%.
Alternatively, an energy recovery snubber circuit may be provided, which recovers most of the energy back to the supply for delivery to the load in a subsequent switching cycle. U.S. Pat. No. 4,130,862 describes a circuit which uses an additional winding coupled to the inductor to achieve this. However, in this approach, the peak voltage seen by the switching MOSFET becomes limited to a defined multiple of the input voltage (2× in this case).
In addition to the above issues, a further drawback of the flyback converter is that power is only transferred to the secondary circuit when the switch is open. The highly discontinuous waveform leads to high RMS currents in the output capacitor which must supply the full output current while the switch is conducting.
FIG. 3 illustrates a so-called “fly-forward” converter topology, in which the functionality of the flyback converter topology is combined with that of a forward converter topology. The fly-forward converter significantly reduces the current stress in the output capacitor, as compared with a flyback converter.
The fly-forward converter resembles a flyback converter, with the addition of a transformer T. Transformer T has a primary winding Ta and a secondary winding Tb for each output. The primary winding T1a of the transformer is connected in series with the primary winding La of the coupled inductor. The/each secondary winding of the transformer is connected via an additional diode DT to the output capacitor C.
When switch S is closed, current flows in both primary windings La and Ta. The voltage induced in Tb causes current to flow to the output capacitor when the switch is closed, whereas the coupled inductor L transfers energy to the output capacitor when switch S is subsequently opened, as described above for the flyback converter.
FIG. 4 shows typical current waveforms for the circuit illustrated in FIG. 3. For the purposes of illustration, a turns ratio which is not 1/1 is assumed so that the current through T1b can be distinguished from the primary current. It can be seen that current flows in the secondary circuit while the switch is both on and off, and can, with appropriate component selection, be made to flow continuously. This lowers the losses and current stress in the secondary circuit considerably.
A drawback associated with the fly-forward converter topology is that the magnetising energy stored in T1 while switch S1 is closed must be dissipated in a snubber, or recovered using an additional circuit.
In the TDK Lambda p-series, an active-clamp power stage is used to enable energy recovery. However, this is a relatively high cost solution which requires 2 power MOSFETs. In JP 8,023,676, a resonant-reset capacitor is used to recover the magnetising energy stored in T1. However, some of the energy recovered by the resonant-reset capacitor is dissipated when the MOSFET switches on. Moreover, the resonant-reset capacitor will typically result in a higher off-state voltage for the MOSFET, which leads to higher on-state losses.
It is an object of the present invention to overcome the problems associated with the prior art.